1. Field of the Invention
The invention relates to hard or softened, stabilized thermoplastic molding compounds based on polyvinyl chloride (PVC) or on polymers containing chiefly vinyl chloride.
2. Discussion of the Background
It is known that chlorine-containing polymers readily undergo thermal breakdown, e.g., when being treated. This leads to undesired coloration, and degradation of mechanical properties. In order to avoid such breakdown, stabilizing agents are added to polymers prior to processing. For PVC and mixed polymers containing chiefly vinyl chloride, the principal stabilizers used are organotic compounds, inorganic and organic lead salts, organoantimony compounds, or combinations of cadmiun- and barium carboxylates. These so-called primary stabilizers frequently have co-stabilizers added to them to improve their effectiveness. The operation of primary stabilizers, co-stabilizers, and their synergy, is described in the literature, e.g., in Nass, L.I., 1980, "Heat Stabilizers", in "Kirk-Othmer Encyclopedia of Chemical Technology", Vol. 12, 3rd Ed., pub. John Wiley & Sons, p.225.
The stabilizers described are quite effective. However, compounds of lead, cadmium, and antimony are not usable for all applications, because of their toxicity. In particular, they cannot be used in food packaging materials or in the medical area. Organotin compounds, many of which are toxicologically acceptable, are unsuitable for wide application because they are too expensive, i.e., they are not readily available and cannot be produced inexpensively.
Thus, there is a demand for stabilizer systems which are comprised of components which are physiologically unobjectionable, inexpensive, and readily available. One might consider using fatty acid salts of calcium and zinc as primary stabilizers, along with possible co-stabilizers such as epoxy compounds, organic phosphites, polyhydric alcohols, or 1,3-diketones. The disadvantage of such stabilizer systems is low shelf life and/or undesirable base coloration of the polymer compound. There are co-stabilizers for zinc-based stabilizer systems, which co-stabilizers improve the initial and long term effectiveness of the primary systems; such co-stabilizers are namely thioacetals and monothioacetals (Ger. OS 35 42 862). These compounds have the disadvantage of disagreeable odor and an undesirable base color of the polymer compound (see Table 2).